The long-term goal of this project is to understand the molecular network through which brassinosteroid (BR) and other hormonal and environmental signals together control plant growth and development. Steroids are important hormones in both plants and animals. BR regulates a wide range of developmental and physiological processes in plants through a signal transduction pathway from cell-surface receptor kinases (BRI1) to nuclear transcription factors (BZR1 and BZR2/BES1), and through crosstalk with other hormonal and environmental signaling pathways. Using a combination of genetic, genomic, biochemical, and proteomic approaches in the Arabidopsis model system, we have made tremendous progress in understanding the molecular details of how BR binding to BRI1 leads to activation of BZR1 through the evolutionarily conserved BSU1 family of PP1-like phosphatases and GSK3-like kinases. Furthermore, we have identified thousands of genomic targets of BZR1 and revealed at molecular level how the BR signaling pathway is integrated with many other signaling pathways to control cell elongation and various specific developmental programs. For this renewal proposal, we plan to continue using the combinatorial approaches to further advance our understanding of the BR signaling network. We will focus on the mechanisms of signal transduction downstream of receptor kinases, signal integration by interacting transcription factors, and integration of additional signals with the BR pathway. We will achieve the following specific Aims: 1) Elucidate the mechanisms by which the BSU family phosphatases transduce signals from different receptor kinases to distinct or overlapping downstream responses; 2) Identify and characterize the mechanism that mediates BIN2/GSK3 degradation and the functions of additional GSK3 substrates; 3) Gain a mechanistic understanding of transcriptional co-regulation through genome-wide quantitative analysis of transcription factors-DNA interactions and proteomic identification of BZR1-associated nuclear proteins; 4) Elucidate the molecular mechanisms through which sugar and target of rapamycin (TOR) regulates the steroid-dependent growth responses. The experiments outlined in this proposal will further advance our understanding of the molecular mechanisms of signaling specificity and crosstalk downstream of receptor kinases and the mechanisms of signal integration at the level of transcriptional regulation in the nucleus. Thus, this study not only is important for plant biology and agriculture, but also can potentially help us understand fundamental mechanisms of signaling and cellular regulation that are relevant to human health.